1. Field of the Invention
This invention relates to a process. More specifically, this invention is directed to solvent induced crystallization (SINC) of amorphous crystallizable polymers.
2. Description of the Invention
As is well known in the art, crystallization of polymers can be achieved by anyone of several processes. One such process which is of primary industrial importance involves thermal induced crystallization of polymeric materials either from the melt or from the glassy phase. Strain induced crystallization, also employed commercially, provides an alternative to the thermal treatment described hereinabove.
Another method of achieving crystal formation in polymers is by induction with appropriate solvents. This is termed solvent induced crystallization (hereinafter, SINC) or liquid induced crystallization. The occurrence of this phenomenon has been known for a considerable time and a number of studies have dealt in detail with several aspects of the so called SINC process. The most recent literature on this topic has involved the study of solvent induced crystallization of polyethylene terephthalate (PET); although earlier investigations also considered the action of solvents on cellulose triacetate and cellulose tributyrate. The effect on polycarbonates has also been previously noted. These early investigations show qualitatively (by means of X-ray diffraction patterns) that solvents are capable of interacting with these polymers and thereby increasing their crystallinity. It has also been illustrated that the nature of the solvent relative to the polymer is an important criterion, since different solvents effect the polymer to varying degrees.
The crystallization of amorphous selenium has been extensively studied and the factors effecting such physical transformation carefully documented, "Selenium", R. A. Zingaro and W. C. Cooper (Eds), Van Nostrand Reinhold Company, 1974 New York, Chapter 3, pp 125-133. It is known, for example, that certain solvents can induce crystallization upon heating with amorphous selenium and that a solvent may be directive for conversion of this amorphous metal to a particular crystalline form, A. P. Saunders J. Phys. Chem. 4, 423 (1900). Even exposure of amorphous selenium to water vapor can induce its conversion to the trigonal crystalline form, Y. S. Chiang, and J. K. Johnson, J. Appl. Phys. 38, 1647 (1967).
In its simplest terms, the process of solvent induced crystallization may be considered as involving the following steps: the solvent is imbibed into or just below the surface of the polymer by diffusion; the solvent polymer interaction causes enhanced mobility of the polymeric segments by interrupting the intersegmental forces; and, if the polymer solvent interaction is strong enough, the polymeric macromolecules rearrange themselves into a more thermodynamically lower energy state--generally the crystalline state. Thus, those solvents which are capable of sufficient interaction with a given polymer may be suspected to induce crystallization without dissolution of the polymer. Clearly, if segmental mobility is favored, (as compared to the dispersion, polar, and hydrogen bonding forces holding the macromolecules together), solvent exposure may lead to complete dissolution of the polymer. Typically, some dissolution will in fact occur at the interface between the pure solvent and the polymer sample due to the large concentration gradient.
The advantages of using solvent induced crystallization, as opposed to thermally induced crystallization, resides primarily in the achievement of crystallization at reduced temperatures. Moreover, in the case of solvent induced crystallization, the solvent-polymer interactions may differ in degree from solvent to solvent thus enabling greater control and variation in the process parameters. It is anticipated that at a fixed temperature, the morphology and final properties of crystallized polymer particles can be readily manipulated by the choice of a particular solvent.
The phenomenological processes involved in solvent induced crystallization appear to lie somewhere in between crystallization from solution and thermal induced crystallization processes. Some researchers regard solvent induced crystallization as resulting from a partial or localized dissolution of the polymer to form a concentrated solution followed by later crystallization of the polymer and ejection of the solvent from the crystal lattice. It is believed that the parameters which are of prime importance in solvent induced crystallization processes involve the intrinsic properties of the polymer, the nature of the solvent employed, the temperature of crystallization relative to the T.sub.g of the polymer, the kinetics of crystallization and the kinetics of solvent diffusion into the polymer. Depending upon how these variables are adjusted and combined, the resultant materials will possess different morphological textures and different material properties.
Most of the techniques disclosed in a literature regarding solvent induced crystallization simply involve immersion of a polymeric sample into a solvent and allowing the sample to remain in the solvent for a prescribed period of time. At the end of this processing sequence, the sample is withdrawn from the solvent, generally dried and the crystalline materials evaluated, see Desai and Wilkes, J. Polymer Sci.: Symposium No. 46, 291-319 (1974) John Wiley and Sons, New York City. As is readily apparent, the techniques employed in the prior art systems which involve solvent induced crystallization are self limiting; that is, as crystal growth occurs on the polymer surface, diffusion of the solvent into the polymer is inhibited. Thus, solvent induced crystallization, as practiced by the prior art, is inefficient and not readily capable of complete conversion of polymer samples to their corresponding crystalline form.
Accordingly, it is the object of this invention to remedy the above as well as related deficiencies in the prior art.
More specifically, it is the primary object of this invention to provide a process for the rapid and efficient conversion of amorphous inorganic and organic polymers to monodispersed crystalline particles.
It is another object of this invention to provide a process for substantially complete conversion of amorphous selenium to the corresponding polymeric, triclinic crystalline form in a rapid and efficient manner.